In cellular networks, such as a cellular network based on the LTE (Long Term Evolution) radio technology specified by 3GPP (3rd Generation Partnership Project), it is typically required that a frequency utilized by a radio receiver/transmitter in a user equipment (UE) matches a frequency utilized by a radio receiver/transmitter in a base station of the cellular network, in the LTE radio technology referred to as eNB (evolved Node B). To meet this requirement, the UE may perform frequency error measurements based on reference signals transmitted by the base station. In the LTE radio technology, these frequency error measurements are typically performed on cell-specific reference symbols (CRS) which are distributed over a wide frequency band of up to 20 MHz.
One aspect of the LTE radio technology specifically addresses Machine Type Communications (MTC) and a corresponding class of UEs, referred to as MTC device, as well as specific features to support efficient MTC, have been defined on both the network side and the UE side. A specific variant of MTC is referred to as NB-IoT (Narrow Band Internet of Things). One target of MTC and NB-IoT is to enable low-cost and/or low complexity radio devices with low-power consumption and extended coverage. This is typically achieved by limiting the MTC or NB-IoT devices with respect to their capability to utilize the full bandwidth and high data rates supported by the LTE radio technology. For example, an MTC device may be operated in a narrow frequency band of 1.4 MHz. This operation is also referred to as narrowband LTE. In the case of NB-IoT (Narrow Band Internet of Things), the utilized bandwidth can be even as small as 200 kHz.
To reduce cost and complexity, NB-IoT and MTC radio devices may use low cost oscillators, e.g., a Digital Controlled Crystal Oscillator (DCXO) or free-running crystal oscillator (XO), as a local oscillator or more generally a frequency reference source for operating the radio receiver/transmitter. However, such low cost oscillators may have more imperfections than more accurate and costly oscillators. For example, the oscillators may be limited with respect to the stability of their output frequency over temperature. Further, the NB-IoT and MTC radio devices may support only half-duplex transmission, which means that they are not capable of receiving and transmitting at the same time.
The NB-IoT technology may also be used to support different coverage ranges, referred to as normal coverage, extended coverage, and extreme coverage. In the case of extreme coverage, a data rate of at least 300 bps can be supported. Typical message sizes of NB-IoT applications are in the range of a few 100 bytes. For example, according to 3GPP TR 45.820, a mobile autonomous reporting (MAR) application has a packet size of up to 200 bytes, which will eventually be conveyed in one or more transport blocks. By way of example, assuming a maximum transport block size of 1000 bits in the uplink direction and 300 bps data rate then it will take around 3.3 seconds to transmit each transport block. Such a condition sets a challenge for meeting specified frequency error requirements, which are typically ±0.1 ppm as, because with increased duration of the uplink transmission, also the risk of higher frequency errors increases, the UE not being able to compensate for its frequency errors, since it is not able to receive DL signals in order to estimate its frequency error. Such frequency errors can for example be introduced by a temperature change, e.g., due to self-heating of a power amplifier during long continuous transmissions. An excessive frequency error can in turn introduce inter-carrier interference (ICI) at the eNB and can significantly degrade transmission performance, e.g., in terms of throughput.
Accordingly, there is a need for techniques that allow for efficiently estimating frequency errors of a reference frequency source used by a radio device operated in half-duplex mode.